Titanium metal has advantages of small density, high specific strength, corrosion resistance, high temperature resistance, non magnetism, non toxicity and the like; and a titanium alloy has a memory function, a super-conduction function, a hydrogen storage function and the like. At present, the titanium metal has been widely applied to military fields such as aerospace and war industry as well as civil fields such as chemical engineering, marine, automotive, sporting equipment, medical instruments, architecture and the like, and is honored as a “future metal”, a “third metal”.
At present, a prevailing production process of the titanium metal is a Kroll method, that is, an aluminothermic reduction method of titanium tetrachloride. Its core process comprises: placing a magnesium metal in a reactor and flushing with argon gas for protection, heating to 800° C.-900° C., and then adding the titanium tetrachloride at a certain speed to react with molten magnesium metal to prepare titanium sponge, wherein a purity of the titanium is about 99.7%. Its metallurgical production process is complicated and cumbersome in flow, and high large in energy consumption and cost, such that its price may not be lowered. The titanium metal is high in price for these reasons, which greatly limits the application of the titanium metal.
In 2000, D. J. Fray of University of Cambridge, UK, proposed a process for producing titanium sponge through cathode deoxidization in molten CaCl2 by using TiO2 as a raw material (WO09963638). Its process has the following characteristics: (1) low electrolysis deoxidization efficiency; (2) complicated deoxidization procedure; (3) relatively high requirement for a purity of a titanium oxide raw material. Accordingly, an industrialized procedure of the FFC process also requires a relatively long period to try to address the above problem, and it is undesirable that the Kroll method is replaced by this method to produce the titanium metal within a short period.
A research group of D. Sadoway of Massachusetts Institute of Technology prepared a liquid titanium metal by electrolyzing a TiO2-containing oxide melt at 1700° C. This process is simple, is capable of realizing continuous production, and produces O2 at an anode, which is free of pollution to an environment. However, since an operating temperature of this process is 1700° C., there is a need for a precious metal material as its anode, resulting in high cost. In addition, liquid titanium prepared by electrolyzing the melt titanium dioxide is deposited on a bottom of an electrolytic cell to be in direct contact with a high-temperature molten salt layer containing oxygen ions, which typically causes a problem of high oxygen content of a product, so far, the oxygen content of the titanium metal obtained by such a method is greater than 2%, which differs too much from a quality requirement of an available titanium metal. Therefore, at present, it is still undesirable that the environmental titanium is directly electrolyzed by such a method.
A research of Okabe and Ono of Kyoto University was as follows: in a CaCl2 molten salt, titanium dioxide was reduced with activated calcium obtained by electrolysis into a titanium metal. It differs from the FFC process of University of Cambridge in that the titanium metal is obtained by reducing TiO2 with a calcium metal obtained by electrolysis, rather than directly by titanium dioxide cathode deoxidization. Also, this process has problems similar to those of the FFC process of University of Cambridge, such as low current efficiency, high oxygen content for product quality, high requirement for a purity of a titanium dioxide raw material, and the like.
In the 1950s, E. Wainer made a research as follows: TiC and TiO which served as raw materials were thermally treated at a high temperature of 2100° C. after mixed to form a solid solution (TiC—TiO), and the solid solution which served as an anode was electrolyzed in a chloride molten salt, he found that a CO gas was emitted from the anode and there was no remaining product (anode mud) in an anode region, and the solid solution might be deposited at a cathode after electrolyzed for a long time to obtain pure titanium. However, there was a need for TiC and TiO as raw materials for a scheme proposed by Wainer, wherein TiO was hardly prepared and controlled, and the solid solution of the TiC and the TiO was prepared under a condition of a high electric arc melting temperature (>2100° C.); thus problems are evident in an actual application.
Y. Hashimoto as a research worker in Japan made a research as follows: excessive carbon and TiO2 which served as raw materials were mixed, and prepared into oxygen doped TiC by employing a high electric arc temperature (>1700° C.), and the oxygen doped TiC which served as an anode was electrolyzed in a molten salt, and deposited at a cathode to obtain pure titanium. However, a preparation procedure of the anode was always dependent on a very high-temperature (>1700° C.) reduction condition, and did not essentially achieve the purpose of extracting titanium at a low cost, and his electrolysis experiments all predominantly used low-oxygen titanium carbide, and a large amount of anode mud was produced due to too high carbon content of the anode, such that continuous electrolysis might not be normally conducted.
MER in USA developed a novel electroreduction process (WO2005/019501). This process is as follows: TiO2 and C were mixed in a stoichiometric ratio, and thermally reduced at 1100° C.-1300° C. to obtain a composite of a low valence oxide of titanium and carbon, and the composite served as a composite anode was electrolyzed in an alkali metal molten salt system to obtain a titanium metal. In this process, the composite anode was a mixed material of the low valence oxide of titanium and the carbon, anode mud and residual carbon might be in an electrochemical dissolution procedure, and thus a problem of short-circuiting between electrodes might be caused as an amount of residual carbon increases and a product might be polluted.
In 2005, D. Hong-Min Zhu of University of Science and Technology Beijing proposed a novel process for winning and smelting clean titanium (ZL200510011684.6), which is as follows: titanium dioxide and graphite which served as raw materials were subjected to carbothermal reduction in vacuum at 1500° C. to prepare a Ti2CO with good electrical conductivity, and the Ti2CO which served as a soluble anode material was electrolyzed in a NaCl—KCl molten salt system at 700° C. to win a high-purity titanium metal, and the high-purity titanium, with carbon and oxygen contents both less than 300 ppm, was obtained on a cathode. A scientific and mechanism problem had been intensively studied in this process, and small-scale intermediate experiments were conducted, which proved its feasibility.
Panzhihua Steel, Sichuan, applied a method for electrowinning a titanium metal from a titanium cyclic molten salt (CN 101519789A) in 2009, which is as follows: titanium tetrachloride which served as a raw material was reduced to a low valence chloride of titanium by using the titanium metal, and then the titanium metal is obtained by molten salt electrolysis. The method had the following problems: prices of the titanium tetrachloride and the titanium metal which served as the raw materials are high, and a reaction rate of reducing the titanium tetrachloride to the low valence titanium is low, which also resulted in high production cost of the titanium. Panzhihua Steel, Sichuan, applied a method for preparing a titanium metal (CN 101914788) in 2010, which is as follows: excessive carbon was directly proportioned when titanium slag is smelted from a titanium concentrate, then nitrogen was introduced to prepare titanium nitride or titanium carbonitride, and the titanium nitride or titanium carbonitride was electrolyed to obtain the titanium metal. In this method, the excessive carbon was proportioned to prepare the titanium nitride or titanium carbonitride. The method had the following problems: (1) due to excessive carbon proportioned in, there was residual carbon in preparing the titanium nitride or titanium carbonitride; (2) carbon therein may be separated out in a form of elemental carbon to become residual carbon during long-term electrolysis when the titanium nitride or titanium carbonitride served as the anode; the residual carbon produced in the above two cases might pollute a quality of a product at a cathode, and easily caused problems of short circuiting between electrodes, low current efficiency, high carbon content of the product, and the like.